PIC internal OSC stability

[max0412]

Does anyone know whether the +/-1% spec for the internal OSC on PIC’s is over the full temperature range or is this spec referring to component to component variations. I’ve looked through the datasheets expecting to find a graph of drift vs temperature but there isn’t one?

The reason I ask is that the closest I can get to 38 kHz IR carrier with a PIC10F200T and external 1.8432MHz external crystal is 38.4kHz.This is similar to the quoted +/-1% spec for the internal OSC. This is still within the 95 percentile of the PNA4602 detectors pass band.

In a nutshell what I’m asking is it worth a buck to use an external crystal in this case or should I save a buck and just rely on the internal RC OSC? Particularly when I’m assuming that each PNA4602 will also vary over temp and component to component.

 

[FvM]

To my opinion, the datasheet specifications, see below for a PIC16F688 are pretty clear. Some commercial products are using internal PIC internal oscillator and factory calibration to set RS232 baud rates, even for a wide temperature range.

 

[max0412]

To my opinion, the datasheet specifications, see below for a PIC16F688 are pretty clear

Well yes it is clear for that particular device .No such graph exist for the PIC10F200T.

I'm building at least 20 transmitters assuming the same applies to my PIC I think I'll stick with the XTAL. This way I know it will be 38.4kHz unit to unit (+/- 20ppm 25C) with a +/- 200ppm MAX over 0-70c. This guarantees I'm within the high 90 percentile of the PNA4602 pass band.

 

[VVV]

That is at room temperature. Over the full temp range it's 5%.
So you will have to look for an 8-pin PIC, such as 12Fxxx, because the 10F does not work wih an external oscillator. That is what I did for a very similar application.

 

[max0412]

So you will have to look for an 8-pin PIC, such as 12Fxxx, because the 10F does not work wih an external oscillator. That is what I did for a very similar application.

Yes I found that out after I bought twenty (Live and Learn). I have somewhere else I can use them though. They are only 50 cents each at NEWARK not bad for what you get. Digikey wants $ 1.35 cut tape.

I’m going to use a PIC12F629T and a 12MHz crystal. With the crystal and the PIC it’s only $1.40 and I can get 37.975 kHz. This is still cheaper and more stable over temperature then a CMOS 556 with film cap and thin film resistors. I found the variations unit to unit quoted for the TLC556 are a tad optimistic. It’s good enough for one – offs but if you start building several units..... I don’t want to be playing with trimmers all day.

I was originally going to just build a discrete oscillator and use Divide by N counter’s but two or three sixteen’s pin IC’s plus at least one logic gate is a bit much.